Which book is good for 9th cbse physics?

Firstly, you don't need to buy any guide in class 9 because the science NCERT is self sufficient at that level. But if you have Great interest in science and want to increase your knowledge for Olympiads and future entrances you may surely buy one, that is :

S Chand.

I have tried NCERT and S Chand, and I found S Chand to deliver a more in-depth knowledge of the topic.

However, if you're aiming for NEET or IIT, or simply wish to understand physics better, I simply recommend you to buy Concepts of physics By HC Verma.

This book will help you crack your entrances and moreover, it is written in a very easy to understand language. It can prove to be a great companion for 9th, 10th, 11th, and 12th.

Even though many will discourage you to try and study from it as they may consider it a bit high-leveled but there's no such thing as 'easy' physics.

There's only Physics.

And HC Verma is always beneficial to have in the long run

Whichever book you buy, study it well. Don't just buy it so it can eat dust